1. Field of the Invention
The present disclosure relates to a capacitive micromachined ultrasonic transducer that can be used in ultrasonic probes and other applications, and also to a method for manufacturing this capacitive micromachined ultrasonic transducer.
2. Description of the Related Art
Ultrasonic diagnosis has recently been appreciated as a technology for early detection of diseases. In this field of diagnosis, one of the promising ultrasonic transmitting and receiving technologies under research is capacitive micromachined ultrasonic transducers (CMUTs), replacing piezoelectric elements. CMUTs are small and lightweight devices and are fabricated by the rapidly advancing micromachining technology. They have an acoustic impedance similar to that of the human body and thus offer better acoustic impedance matching than known piezoelectric devices. They are also advantageous in many other ways, for example, a broad frequency band in liquids.
PCT Japanese Translation Patent Publication No. 2006-516368 discloses a method for fabricating a CMUT, and this method involves the use of a single-crystal silicon vibrating membrane formed on a silicon substrate by bonding them or other suitable means. More specifically, an oxide film is formed on the silicon substrate by thermal oxidation, the resulting thermal oxide film is partially removed, and then the remaining portion of the thermal oxide film and a piece of single-crystal silicon are bonded. This piece of single-crystal silicon is used as the vibrating membrane, with the space formed by the partial removal of the thermal oxide film as a cavity.
The above patent publication mentions that the manufacturing method disclosed therein may include a second thermal oxidation step. After the removal of the thermal oxide film to expose a portion of the silicon substrate and thereby to form the cavity, the exposed surface of the silicon substrate is thermally oxidized again and thereby coated with an insulating oxide film.